: The formation of new vessels, angiogenesis, depends critically on the interactions of endothelial cells with each other and with the underlying extracellular matrix. Consequently the molecules that mediate these cell-cell and cell-matrix adhesive interactions are of fundamental importance to this process. One important cell adhesion molecule on endothelial cells is PECAM-1 where it concentrates at intercellular junctions. Recently, data has implicated PECAM-1 in in vivo angiogenesis. However its mechanisms of involvement and the regulation of these processes are not known. Therefore, the overall aim of this proposal is to characterize the mechanism(s) of PECAM-l's involvement in angiogenesis. Preliminary in vitro data suggests that angiogenic factors alter the surface distribution PECAM-1 and that PECAM-1 is involved in endothelial cell migration and tube formation. As PECAM-1 is able to bind to itself and non-PECAM-1 ligands, the hypothesis is that distinct PECAM-1-dependent adhesive ligand interactions, regulated by angiogenic factors, are involved in the initial migration of endothelial cells into perivascular matrix and their subsequent organization into vascular tubes.To test this hypothesis the following are proposed: (1) Determine the effect of angiogenic factors on endothelial PECAM-1, by studying the effect of angiogenic factors, either alone, or in various combinations, on endothelial PECAM-1 surface distribution, tyrosine phosphorylation, cytoskeletal association and alternative splicing; (2) Define the involvement of PECAM-1-dependent homophilic and heterophilic adhesion in endothelial cell migration and tube formation, using assays of endothelial cell migration and tube formation in the presence of antibodies known to block heterophilic or homophilic adhesion and cellular transfectants expressing mutant forms of PECAM-1 that mediate exclusively heterophilic or homophilic adhesion, and; (3) Determine the involvement of PECAM-1 and its possible mechanisms of action during in vivo human angiogenesis, by studying the effect of anti-PECAM-1 antibodies with documented effects on endothelial migration and tube formation on the angiogenic response of human tumors and wounds in human skin transplanted on SCID mice.Because a number of pathological processes are mediated by stimulated or impaired angiogenesis, we believe that as our understanding of the role of PECAM-1 in vessel formation increases, new insights will be gained that will point to novel therapeutic approaches to a variety of diseases.